Data storage on magnetic tape is well known and tape recorders have been used to record data tracks on magnetic tape. Traditionally, data is recorded in a plurality of parallel data tracks on the magnetic tape. The read/write head is then positioned relative to the tape path by moving the head to different track positions as desired. In such a system, the tape tracks are generally sufficiently wide and separated to facilitate reliably reading and writing the data.
In one approach, the read/write head may be positioned at a predetermined fixed point, relative to the magnetic tape path, and the data tracks are intended to accommodate variations of recording track location and tape locations as the tape feeds past the head. Historically, this accommodation has been accomplished by providing sufficient track width and data track separation on the magnetic tape to permit the read/write head to remain positioned over the designated track and at the same time not read magnetically recorded signals from an adjacent track. This arrangement of track width and track separation is intended to accommodate deviation of the track location from the design norm either due to being recorded on a first recorder and played or rerecorded on a second recorder or due to the wander of the tape as it is spooled past the read/write head from one spool to the other spool of a cartridge.
However, the definition of the magnetic read/write head and the track widths and separations can limit the number of data tracks that may be recorded on any given width of tape. Reliance solely on the track width and track separations for reliable read/write operations can result in a significant waste of magnetic tape surface and thus limit the data density on the tape. However, as the track width and the read/write head width narrow in an effort to increase the data capacity of a given tape area, any misalignment of the head with the track may lead to read/write repeatability failures and lost data. Thus, the resolution of the head placement mechanism and the precision of the placement of the tape relative to the read/write head can be limiting factors affecting the recording density of data on the magnetic tape surface in tape drives having static read/write heads.
More recently, tape drive systems have used a track following servo system for moving the tape head in a lateral direction to follow lateral movement of the longitudinal tracks as the tape is moved in the longitudinal direction. The track following servo system may employ servo tracks on the tape which are parallel to the data tracks, and employ servo read heads to read the servo tracks to detect position error and thereby position the tape head at the data tracks and follow the data tracks. This allows the data tracks to be placed closely together and increase the number of data tracks.
The tape is typically contained in a cartridge of one or two reels, and the tape is moved between a supply reel and a take up reel. The reels typically have runout causing the tape to move laterally as the tape is moved longitudinally. Tape guides are often provided to limit the amplitude of the lateral movement of the tape so that it does not exceed the lateral movement capability of the track following servo system.
In high track density tape storage devices, a compound actuator may be used in a track following system. The compound actuator typically includes a coarse actuator which usually is a stepper motor, and a fine actuator which frequently has a linear high bandwidth, and a limited range of travel. Thus, a compound actuator can have both high bandwidth and a large working dynamic range.
In such a compound actuator system, a magnetic read head gap may be placed at a position relative to a servo track on the tape. Then the read head gap is further moved to detect the edge of the servo control track (servo track) recorded on the tape. The read head will provide signals which may be used to indicate the head location relative to the servo track. By using these signals as a basis, the servo control then may produce a positioning signal to drive a servo positioner. The servo positioner moves the read head, causing the read head to track or follow the edge of the servo control track which has been previously recorded on the tape.
The transient response of the tape head track following servo system typically comprises a high bandwidth for a very limited lateral movement, called “fine” track following, for allowing the tape head to accurately follow small displacements of the tape. Larger movement of the tape head is typically conducted as “coarse” track following, which is also employed to shift the tape head from one set of tracks to another set, and is typically conducted at a slow rate. However, it is appreciated that the occurrence of a lateral transient shift, can be so rapid that neither the fine track follower nor the coarse track follower is able to respond sufficiently. As a result, the tracking can become so large that writing may be stopped to prevent overwriting an adjacent track and to insure that the tracking error on read back is not so large as to cause a readback error.
An example of a compound actuator is described in coassigned U.S. Pat. No. 5,793,573. FIG. 10 shows one example of an existing fine actuator 1000 having a tape head (not shown) suspended by a beam 1015 between dual flextures 1020, 1030. A voice coil motor (VCM) disposed in a base 1040 is coupled to the lower flexture 1030 and to the beam 1015 supporting the head between the flextures 1020, 1030. Current applied to the voice coil motor causes the motor to displace the lateral location of the head relative to the tape, depending upon the direction and magnitude of the applied current.
The flextures 1020, 1030 each have a spring constant which biases the beam 1015 and the head to a central neutral location in the absence of current applied to the voice coil motor. To counteract a natural resonance frequency of the spring constant exhibited by the flextures 1020, 1030, a damping mechanism such as a damping fluid may be provided in the base 1040.